I've been working with Opus 4 on ultrathink quite a bit recently and did some quick tests with Sonnet 4.5, I'm fairly impressed, especially with its speed but I did feel it was a lot less strict with my rules, existing patterns, etc. compared to Opus 4.

Maybe it's better with a better CLAUDE.md structure? I don't use those a lot, just telling Opus to think got 'good enough' results I guess. Not sure.

I hope there's an Opus 4.5 coming out soon too. In the meantime I'll see if I can get to do better with some extra prompting or I'll go back to Opus of if I don't need the speedier responses.